Sharpness emphasis signal processing of image reproduction by means of an image reproducing system is carried out in the following manner. At first, a sharp signal M and an unsharp signal N are obtained by respectively using a beam for analyzing the density value of a centered pixel and a beam of greater diameter for analyzing the density values of the pixels surrounding the centered pixel. Secondly the difference between the signals is computed and is multiplied by a coefficient q to produce a differentiation signal .DELTA.P=q(M-N). Then the differentiation signal .DELTA.P is added to the sharp signal M to produce an image signal M+.DELTA.P having undergone sharpness emphasis (refer to FIG. 5).
The above-mentioned procedure is of course for an analog data processing, and requires both an optical system for obtaining the unsharp signal and electronic devices for processing the signal obtained therefrom, in addition to requiring the same equipment for the sharp signal.
Furthermore, the aperture size of the unsharp beam must follow that of the sharp beam. The aperture of the latter varies in accordance with the variation of the resolution power thereof, determined in accordance with a magnification ratio in reproducing images. Therefore, several sets of apertures for the sharp beam and the unsharp beam must be provided for the variation of resolution power.
In order to solve the above drawbacks, several methods are disclosed in U.S. Pat. No. 4,319,268 and U.S. Ser. No. 573,967, in both of which a sharpness emphasis signal is obtained digitally.
In these methods, at first an image data is obtained by analyzing a plurality of scanning lines of an original by means of an image input means. The data are stored into a buffer memory, and then arranged in order of scanning lines. Secondly a number of pixels located in a certain area (for example, an area comprising 3.times.3 pixels in the main and the sub-scanning directions) are subjected to a process for obtaining the sharp signal M and the unsharp signal N. The sharp signal M is obtained from the density value of the centered pixel (called a "center pixel" hereinafter), and the unsharp signal N is obtained by averaging the density values of the pixels surrounding the center pixel (each of which is called a "surrounding pixel") in a circuit for summing products, as hereinafter described. Then a differentiation signal .DELTA.P is obtained by detecting the difference between the signals, and is added to the sharp signal M to obtain an image signal M.times..DELTA.P having undergone sharpness emphasis. The aforesaid circuit for summing products computes data according to an expression: ##EQU1## wherein a.sub.j is a weight coefficient to be given to the surrounding pixel I.sub.j (which may depend on the weight coefficient given the center pixel I.sub.0), while V.sub.Ij is the density value of the surrounding pixel, n is the number of the surrounding pixels submitted to be processed at one time, and K is a coefficient defined as follows.
Properly, the sharp signal M (the density value V.sub.I0 of the center pixel I.sub.0) and the unsharp signal N (the density value V.sub.Ij of the surrounding pixel) must agree with each other when the area subjected to the process is not a boundary between constituent images but an area of constant density distribution. In order to make them equal, the value ##EQU2## is multiplied by the weight coefficient K.
Nevertheless, this method has a drawback that a multiplier for performing a computation a.sub.j .times.V.sub.Ij must have a capability of dealing with a signal of twice as many bits as each of the values a.sub.j and V.sub.Ij, which results in an increase in production cost for the device.
Supposing the computation a.sub.j .times.V.sub.Ij is restricted to be performed in a limited number of bits, for example, in 8 bits, then, since the computation ##EQU3## must be performed in a greater number of bits, such an effort for limiting the bit number becomes meaningless.
Furthermore, in order to perform the aforesaid computation ##EQU4## in a practical number of bits, for example, in 8 bits, the number of bits used for expressing the computation result of a.sub.j .times.V.sub.Ij and each of the subsequent additions must be reduced. During the computation process, an accidental error may be produced, and may provide an obstacle to obtaining the accurate unsharp signal.
As mentioned before, the sharp signal M and the unsharp signal N must agree with each other when they are obtained from an area of constant density distribution where the density values of the center pixel and any of the surrounding pixels are the same. In other words, the differentiation signal .DELTA.P must be zero. However, an accidental error as above mentioned may disturb the theoretical relation between the signals, so that the differentiation signal .DELTA.P does not become zero. The differentiation signal .DELTA.P obtained under the influence of the error is to be added to the sharp signal (the density value of the center pixel) as DC (Direct Current) noise, which undesirably modifies the reproduction image recorded thereby.
This kind of drawback of course exists in a conventional method in which a sharp signal M and an unsharp signal N are obtained in an analog fashion. That is, since optical systems for obtaining the sharp signal M and the unsharp signal N are independently provided, the correspondence between both the signals is not always maintained, which fact also requires an adjustment effort to be added thereto.